The right of priority is claimed based on Federal Republic of Germany Priority Application 101 54 679.3, filed Nov. 7, 2001, the entire disclosure of which, including the specification, drawings, claims and abstract, is incorporated herein by reference.
The invention relates to a method for operating a code reader and to a code reader for carrying out this method.
Code readers use various principles for reading a code, such as, for example, a one-dimensional bar code or a two-dimensional matrix code.
In code readers which operate on the scanner principle, the code is read by moving a light beam, referred to as a xe2x80x9cmoving beam,xe2x80x9d over the object to be scanned, for example, a label with a barcode. A laser beam is typically used as the light source due to its good focusing and intensity characteristics. The light reflected and/or re-emitted from the object is received by a light receiver, and is converted to electrical signals. The barcode information is derived from the amplitude information contained in the electrical signals.
Two-dimensional codes are generally read using code readers which operate on the camera principle. In this case, the object to be scanned is illuminated with a light fan, which has a cross section in the form of a line, or with area illumination. The light which is reflected and/or re-emitted from the illuminated object is imaged on a one-dimensional or two-dimensional receiver array, such as a charge coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) line or matrix sensor. The code information can be obtained by evaluating the differences between the reception amplitudes of the various received pixels.
In all code reader applications, incorrect amplitude information acts as a disturbance variable in the identification of the code information. When using laser light, an additional disturbance variable occurs, i.e., xe2x80x9cspeckle noisexe2x80x9d as described in U.S. Pat. No. 6,082,619. When laser light illuminates a white surface, a speckle pattern can be produced in the light spot due to the high coherence of the laser light and to interference between the light waves which are scattered at various points on the white surface. The speckle noise due to this speckle pattern acts as disturbing amplitude noise in the light receiver.
In another context, namely for the optical reproduction of images using a projection method based on lasers, it is known that a speckled pattern occurring during the imaging process may be reduced by pulsing the laser light source (see DE 199 24 519 A1). Pulsed laser light has a shorter coherence length, so that the formation of speckles is reduced.
According to one aspect of the invention, there is provided a code reader. The code reader comprises: a light transmitter having a laser light source adapted for transmitting laser light to illuminate an object having a code; and a light receiver arranged for receiving reflected or re-emitted light from the object and for producing electrical signals corresponding to the received light; and a modulation apparatus that modulates the laser light source such that amplitude-modulated light is transmitted from the laser light source to the object.
According to another aspect of the invention, there is provided a method for operating a code reader. The method comprises: modulating a laser light source to provide amplitude-modulated laser light; transmitting the amplitude-modulated laser light from the laser light source to illuminate an object having a code, wherein the amplitude-modulated laser light reduces the speckle noise of the light illuminating the object; and receiving reflected or re-emitted light from the object by a light receiver.
Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying figures of drawing.